The subject matter disclosed herein relates to auxiliary power units (APUs) for aircraft. More specifically, the subject disclosure relates to an electrical starter torque limiter for an APU.
Aircraft engines have large heavy rotors that must be accelerated to a high rotational speed in order to provide sufficient air compression for self-sustaining operation. An APU provides the power required to get the rotors turning at the required speed. First, however, the APU is started by an APU starter, which is a direct-current (DC) electric motor with power supplied by a battery, accumulator, or external power source, such as a ground power unit. Once the APU accelerates to full speed, it can provide enough power to start the aircraft's main engines, either by turning an electrical generator or a hydraulic pump, or by providing compressed air to an air turbine of the starter motor.
When starting the APU, power is supplied to the DC motor and a clutch which engages the APU to begin its operation. Typically, power is supplied to the DC motor and the clutch from the same electrical line, so that when the DC motor is activated, the clutch is simultaneously activated. The result is that the DC motor accelerates to speed before the clutch is completely engaged, and causes a high torque shock on a starter shaft when the clutch engages the shaft. The torque shock can cause damage and/or breakage of the shaft. The torque shock can also result in damage to an APU gearbox that is driven by the starter shaft. Rapid acceleration of the DC motor after the clutch is engaged may lead to an over-torque condition at the APU gearbox, potentially resulting in damage to the APU gearbox.